Frank Marcos

A New Empirical Thermospheric Density Model JB2008 Using New Solar and Geomagnetic Indices

Abstract : A new empirical atmospheric density model, Jacchia-Bowman 2008, is developed as an improved revision to the Jacchia-Bowman 2006 model which is based on Jacchia s diffusion equations. Driving solar indices are computed from on-orbit sensor data are used for the solar irradiances in the extreme through far ultraviolet, including x-ray and Lyman-alpha wavelengths. New exospheric temperature equations are developed to represent the thermospheric EUV and FUV heating. New semiannual density equations based on multiple 81-day average solar indices are used to represent the variations in the semiannual density cycle that result from EUV heating. Geomagnetic storm effects are modeled using the Dst index as the driver of global density changes. The model is validated through comparisons with accurate daily density drag data previously computed for numerous satellites in the altitude range of 175 to 1000 km. Model comparisons are computed for the JB2008, JB2006, Jacchia 1970, and NRLMSIS 2000 models. Accelerometer measurements from the CHAMP and GRACE satellites are also used to validate the new geomagnetic storm equations.

Magnetic storm response of lower thermosphere density

Measurements of atmospheric density near 200 km from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment are used to delineate the temporal, seasonal-latitudinal, and day/night dependences of the response to magnetic storm-related perturbations in high-latitude energetic inputs. Five periods of geomagnetic activity are analyzed and yield consistent results which can be interpreted within the framework of recent thermosphere-ionosphere simulations by Fuller-Rowell et al. [1995]: In response to a change in magnetic activity level from quiet (Kp ≈ 1–2) to active (Kp ≈ 4–7) conditions, an increase in daytime (1030 LT) density of order 50–70% occurs between 60 and 80° geographic latitude in the summer hemisphere, with about half the maximum response in the winter hemisphere. This difference is mainly due to the difference in ionization/conductivity levels (and hence joule heating rates) between the hemispheres. On the dayside, penetration of the disturbance at about the 50% intensity level is realized at the equator, whereas in the winter hemisphere equatorward penetration is much weaker. These effects are connected with the prevailing solar-driven circulation; the net summer-to-winter meridional flow facilitates equatorward advection of the disturbance bulge in the summer hemisphere but hinders advection in the winter hemisphere. In both hemispheres the daytime component of the solar-driven diurnal circulation tends to oppose equatorward penetration to the same degree. However, on the nightside (2230 LT) penetration at nearly the 100% level of both summer and winter disturbance bulges are realized to within 20° of the geographic equator. This behavior is associated with the equatorward advection in both hemispheres consistent with the nighttime component of the solar-driven circulation. Comparisons with the MSISE90 model [Hedin, 1991] show it to capture the salient features of the daytime behavior but exhibits little day/night asymmetry, in contrast to the experimental results.

The influence of geomagnetic and solar variabilities on lower thermosphere density

Abstract Atmospheric density measurements near 200 km from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment are analyzed for geomagnetic and solar flux variability effects. Data from the SETA experiment, onboard two satellites, are available for the periods of May to November 1982, and July 1983 to March 1984. The data utilized the span ±79.5° latitude, and are available for both day (1030 LT) and night (2230 LT). Annual and semiannual density variations are removed and regression analyses are performed on the residuals using a series of lagged 3 h K p indices to determine and remove geomagnetic fluctuations. Densities are found to increase by as much as 134% in response to an increase in the K p index from 1 to 6. Monthly curves are generated for the K p regression coefficients to delineate seasonal-latitudinal and day/night dependences, which reflect the effects of mean meridional advection of disturbances from high to low latitudes. Further analyses are performed comparing measured densities with MSISE-90 predictions. Results show that the model is able to capture many of the prominent features, but does not fully predict the level of variability for the individual disturbance periods analyzed. After the geomagnetic effects are removed, the residual densities are interpreted in terms of solar flux variability. The daily-averaged SETA density residuals are strongly correlated with long-term solar flux variability, and exhibit a much greater dependence on the 27-day solar rotation period than MSISE-90 predictions. Variations in residual density of the order of 10–20% occur in association with day-to-day and 27-day solar flux variations. The MSIS model does not accurately predict the magnitude of these short-term density variations in response to solar activity.

Accuracy of Earth's Thermospheric Neutral Density Models

[Abstract] Atmospheric drag remains the dominant uncertainty for low altitude satellite precision orbit determination . Empirical models are used to estimate satellite drag. Model accuracies have shown little improvement in the past 35 years. A new Ja cchia -Bowman 2006 (JB2006) empirical model has been developed as part of the Air Force Space Command’s High A ccuracy Satellite Drag Model (HASDM) program. Significant new model features of JB2006 are solar indices based on satellite EUV and FUV sensor s and an improved semiannual variation. This new model is compared to historic models vs altitude, latitude, local time , day of year and solar and geomagnetic conditions . Data are from a unique high accuracy set of thermospheric neutral densities with one -day r esolution, obtained from tracking of 38 satellites. The e valuation is carried out for the period 199 7 to 2004 , when the specific solar indices for JB2006 were available . The results provide improved understanding of quantitative relations between current s olar inputs and t he response of the thermosphere . New formulations incorporated into the JB2006 lead to a capability to more accurately specify thermospheric density .

Thermospheric Space Weather Modeling

We review impacts of satellite drag and describe past, current and future capabilities designed to meet evolving operational requirements. Historically, thermospheric research has been data starved. Thus, from the early space age to the end of the 20th century little progress was made in satellite-drag modeling. This condition improved greatly with the development of empirical assimilative models and recent availability of comprehensive drag measurements. The resurgence in orbital drag analyses to specify thermospheric densities has been particularly useful for addressing input requirements of assimilation models as well as their development and validation. With the new Jacchia-Bowman 2006 model the status of empirical modeling improved significantly. It builds on the expanded satellite drag database and incorporates improved estimates of solar flux changes as well as semiannual and local time variations of the thermosphere. However, magnetic storm representations of Jacchia-Bowman 2006 are similar to those used in other current models. Satellite-borne accelerometers and optical sensors now provide complementary spatial and temporal capabilities that permit monitoring the thermosphere over a wide range of altitudes under most solar and geomagnetic conditions. Long-standing shortfalls during periods of high geomagnetic activity are now being attacked with these data and through new analyses of solar wind and IMF measurements, correlations with magnetosphere-based magnetic indices and emerging theoretical tools. These advances in understanding thermospheric coupling during magnetic storms will be incorporated into empirical model upgrades. The analyses of new data sets joined with on-going research on physical thermosphere-ionosphere- magnetosphere coupling processes support the pursuit of our ultimate goal, an assimilative and predictive operational model of thermospheric neutral densities.

Towards Next Level Satellite Drag Modeling

Orbital drag errors adversely impact many space missions including providing collision avoidance warnings for manned spaceflight and other high-value assets, accurately cataloging of all orbiting objects, predicting reentry times and estimating satellite lifetimes, on-board fuel requirements and attitude dynamics. Uncertainties in neutral density variations are the major satellite drag limiting factor for precise low-Earth orbit determination at altitudes below about 700 km. We review current efforts in empirical and theoretical models dedicated to meeting evolving stringent operational satellite drag requirements. New data sets from orbital drag, satellite-borne accelerometers and remote sensors now provide unprecedented capabilities for modeling thermospheric variability vs altitude, latitude, day of year, local time and solar-geomagnetic conditions. This effort is greatly enhanced by an AFOSR-supported Multi-University Research Initiative. Scientific community efforts are providing steady, previously unattainable, progress supporting Air Force research for an accurate assimilative and predictive operational first principles satellite drag model. There remains a critical need for comprehensive measurements of the thermosphere and relevant heating inputs.

Latitudinal and Diurnal Variations of Neutral Density for Quiet Geomagnetic Conditions

An extensive database of CHAMP neutral density measurements during 2001-2005 is used to re-examine latitudinal and diurnal variations of the exosphere temperature. Exosphere temperatures were derived from the measured neutral densities based on the hydrostatic assumption. A linear regression analysis was conducted to model exosphere temperatures during periods of magnetic quiet times. To improve the previous neutral temperature models, we generalize the latitudinal and diurnal dependences of temperature on solar flux. When compared with the Jacchia 1971 model (J71), the new model shows more detailed diurnal variation. Our results indicate that dayside temperatures extend into the evening sector with small gradients until midnight. In the post midnight sector temperature drops sharply to reach a minimum near 02:00 local time and recovers gradually to daytime values near local noon. The results provide an improved understanding of thermosphere responses to solar flux and more accurate specification of thermospheric densities.

In-Situ O/N2 Ratios from the AFRL Mass Spectrometer on the TacSat-2 Satellite

The Atmospheric Density Mass Spectrometer (ADMS) which flew onboard the TacSat-2 satellite in 2007 has gathered the first in-situ composition measurements of thermosphere in 25 years. ADMS provides a unique contribution to our understanding of the thermosphere because it provides the only localized source of neutral composition data. O/N2 ratios were accurately derived from the count measurements and compared with the NRLMSIS model. These data have been provided to the Defense Meteorological Support program to support the validation of sensor algorithms for ultraviolet remote sensing applications such as the Special Sensor Ultraviolet Spectrographic Imager (SSUSI). AFRL was also asked to estimate column O/N2 ratios from the ADMS data. One method using the High Accuracy Satellite Drag Model (HASDM) total densities and temperature profiles gave column densities in good agreement with selected passes of the GUVI (Global Ultraviolet Imager) remote ultraviolet limb measurements from the TIMED (Thermopshere-Ionosphere-Mesosphere Energetics and Dynamics) satellite.